.243 hands down, especially since your .308 is set up for 180grs, even with 150grs, I'd much prefer a .243 with 90-100grs.

I'd be plenty confident of a .243 out to 500yds, I've always used my .270, but friends with .243 using 95gr still passed through completely, using nosler partitions, I'm looking to get a .243 specifically for this purpose.